The crash marks the first time Google’s car was partially to blame for an accident.

Until recently, Google’s self-driving cars had been involved in 17 minor collisions—none of which resulted in major injuries, and none of which were the fault of the autonomous vehicle.

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That changed earlier this month, when a self-driving Lexus SUV—one of the cars in Google’s fleet—hit a public bus in Mountain View, California, Re/code reports.

The accident occurred when Google’s car swerved to avoid sandbags on the road, causing it to collide with an approaching bus. The driver allegedly saw the bus coming, but thought it would slow down to accommodate Google’s car. Since the bus was only moving at 15 miles per hour, the accident did not result in injuries; Google’s car was traveling at two miles per hour and was still operating autonomously when the crash took place. (During previous incidents, Google’s drivers had turned on the car’s manual mode.)

Google made public a section from its February self-driving car report that discussed the accident, in which the company said “we clearly bear some responsibility, because if our car hadn’t moved there wouldn’t have been a collision.”

Below is Google’s full statement:

Our test driver, who had been watching the bus in the mirror, also expected the bus to slow or stop. And we can imagine the bus driver assumed we were going to stay put. Unfortunately, all these assumptions led us to the same spot in the lane at the same time. This type of misunderstanding happens between human drivers on the road every day.

This is a classic example of the negotiation that’s a normal part of driving—we’re all trying to predict each other’s movements. In this case, we clearly bear some responsibility, because if our car hadn’t moved there wouldn’t have been a collision. That said, our test driver believed the bus was going to slow or stop to allow us to merge into the traffic, and that there would be sufficient space to do that.